THURSDAY, July 26 (HealthDay News) -- Four people attending an Indiana county fair this month came down with flu traced to pigs, U.S. health officials report.

This outbreak of influenza A (H3N2) brings the total number of people infected with this flu to 17 since last August, the U.S. Centers for Disease Control and Prevention said.

This strain of flu, commonly carried by swine, can infect humans who come into contact with infected animals. But transmission to humans has been rare, according to the agency.

"Transmission of the virus happens," said Dr. Michael Jhung, a medical officer in CDC's influenza division. "But if you think about the numbers, if you think about influenza in general, these are uncommon events."

These strains of flu have only a limited ability to be passed from person to person, Jhung said. So far, this strain only passes from one person to one other person, so it is not very contagious.

"What we worry about is this virus or any flu virus getting the ability to transmit efficiently from person to person," Jhung said. "If that happens, you get sustained transmission and we would be worried about that."

Because flu activity is unpredictable, the CDC keeps tabs on new strains and cases as they arise, to guard against widespread contagion, Jhung explained.

A previous case of this flu was seen in March, but these four new cases are the first reported since then, he said.

The report was published in the July 27 issue of the CDC's Morbidity and Mortality Weekly Report.

Genetic analysis confirmed that all those infected had the same flu. None of the four infected people needed hospitalization and all recovered, according to Jhung.

The flu's severity was about as bad as regular seasonal flu, he noted.

The four patients were either exhibiting pigs or were family members in close contact with the pigs.

To avoid contracting this flu, the CDC advises people to limit their contact with pigs and avoid contact with sick swine. People who have contact with these animals should take precautions such as washing their hands, not eating or drinking in areas with swine and controlling their cough.

Most flu starts in swine or birds and spreads to people. In 2009, a type of H1N1 flu from swine resulted in a worldwide pandemic.